The present invention relates to toner (developer) supply control in electrostatographic printing/digital copying machines in which an electrostatic latent image is formed on an imaging member by a printing head and is subsequently developed with toner. The imaging member may, for example, be a photoreceptor belt and the printing head may be a laser device which directs a laser beam at the photoreceptor for imagewise discharge thereof.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent image on the photoconductive member. The toner attracted to a donor roll is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
To maintain print quality over the course of a job, toner concentration must be maintained during the job. This usually means adding toner to the developer housing in a controlled fashion during the entire run.
In a digital xerographic engine, the number of pixels printed can be roughly correlated to the amount of toner to be used, and hence the amount of toner which should be dispensed to maintain proper toner concentration. Printing machines in which that approach is adopted are described in U.S. Pat. Nos. 3,409,901, 4,847,659 and 4,908,666. Noted by the UK PO as relevant to the parent UK application is Canon U.S. Pat. No. 4,468,112, issued Aug. 28, 1984 to A. Suzuki, et al.
Also, particularly noted is U.S. Pat. No. 3,873,002 re the Xerox Corporation "6500" color copier toner dispensing control system.
Following is a discussion of additional prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,204,698 granted to LeSueur et al on Apr. 20, 1993 discloses an electrostatographic laser printing/digital copying machine in which a latent image is generated on a circulating imaging member in accordance with digital image signals and subsequently developed with toner, the number of pixels to be toned is used as an indication of the rate at which toner is being depleted from the developer mixture. The device for dispensing fresh toner to the developer mixture is operated in dependence on the number of pixels to be toned so that there is a pre-established relationship between the pixel count and the length of time for which the dispensing device is in operation. If the efficiency of the dispensing device falls, the pre-established relationship is adjusted so that the toner density in the developed images remains constant. If a predetermined level of adjustment is reached, it is taken as an indication that the supply of toner in the printer is low, and should be replenished.
U.S. Pat. No. 5,402,214 granted to Thomas A Henderson on Mar. 28, 1995 discloses a toner concentration sensing system for an electrophotographic printer which system controls the concentration of the toner in developer mixture in an electrophotographic printer. Toner is applied on a test patch on a charge-retentive surface in a manner consistent with a desired toner density on a test patch, and the actual toner density on the test patch is measured. The charge applied to the charge-retentive surface is then adjusted in response to the measured actual toner density to obtain the desired toner density on a subsequent test patch. The change in charge applied to the charge-retentive surface is used to detect a shortage of toner in the developer.
Xerographic development processes which employ donor rolls, such as Hybrid Scavengeless Development, (HSD) can exhibit a print defect when localized high toner consumption depletes the available toner in one part of the donor roll surface, and the system is unable to replenish the depleted toner in one revolution of the roll. This leads to a repetitive, periodic, gradually declining "ghost" image disturbance propagating in the process direction behind the area of high consumption; this is termed the Reload Defect (RD).
One known strategy to minimize the reload defect is to bias the Toner Concentration (TC) operating point toward the high-TC side of the latitude window. This is effective on a single machine in the short term, but increases the failure frequency in a population of machines and leaves less latitude for long-term effects such as sensor drift, developer contamination, and aging of mechanical components. This strategy can also contribute to excessive dirt generation and resultant machine contamination, most likely requiring manual intervention at setup to determine a unique toner control setpoint for each machine.
A preferred strategy is to operate the xerographic process consistently at a TC level just high enough to prevent customer perception of the reload defect. This optimum TC value will differ from housing to housing due to differences in developer flow, mechanical spacings, electrostatics, developer age and state, and operating environment.
It would be desirable to effect "constant reload level" control by measuring the reload defect level during or just after customer usage, and automatically adjusting the TC setpoint to keep the system at its optimal operating point. This strategy will automatically compensate for long-term effects such as TC sensor drift and contamination, developer aging and contamination, and wear of mechanical components.